DIRECTIONAL SENSITIVITY OF SINGLE OMMATIDIA IN THE COMPOUND EYE OF LIMULUS

Abstract

Lateral eyes of Limulus were mounted on a circular stage of a dissecting microscope so that the eye''s orientation in azimuth could be changed while action potentials of single responding units of the optic nerve were recorded oscillographically. By covering all but certain facets with opaque paint, responses of axons from exposed ommatidia in the center of the eye, where the optical axes were normal to the eye surface, could be recognized. In response both to brief flashes of light and to continuous illumination, discharges could be obtained from a single unit in the optic nerve over a wide range of incident angles. Using a response of 4 nerve impulses resulting from 1-second flashes every 15 seconds as the threshold, high sensitivity was found to be centered within 10-20[degree] of the optical axis, although responses to reasonable light intensities were obtained 80-90[degree] from the optical axis. The directional sensitivity appeared similar in both the dorsoventral and the antero-posterior planes of the eye. Relation of the angular sensitivity of an ommatidum to visual acuity is discussed.